Looks like certificate Files cant be found or seen by slapd,
you should check the used Path of the TLS-* Directives in the slapd.conf
Next, check the Permissions of the Certificicates an the Path, all must be
readable by the user your slapd is running with.

On the other side, you should upgrade to a newer version.
OL 2.2.13 is really outdated.